Voltage divider

ABSTRACT

An apparatus relating to a voltage divider having one or more Kelvin connection taps, including first and second spaced terminals and a current carrying resistor between the first and second terminals. A third terminal is spaced from the resistor and the resistor has at least one conductive but non-current-carrying peninsula extending from a junction on the side of the resistor and connecting with the third terminal to produce a Kelvin connection tap. A cut is made only in the non-current-carrying peninsula along the path having a component parallel to the side of the resistor from which the peninsula extends and shading the junction of said peninsula and the side from the third terminal, to adjust the interconnection of the third terminal with the resistor and vary the voltage output without varying the resistance of the resistor. An extension of this cut may be made parallel to the edge of the peninsula to further increase the adjustment.

FIELD OF INVENTION

The invention relates to a voltage divider having one or more Kelvinconnector taps, and more particularly to such a voltage divider in whichthe tap or taps are formed of a non-current-carrying peninsula extendingfrom a current-carrying resistor.

BACKGROUND OF INVENTION

Single and multiple tap voltage dividers, thick film, thin film andmonolithic, are widely used in electronic equipment. In many cases thesedivider taps constitute Kelvin connections, i.e. connections in which noor essentially no current flows. Such dividers are used in manyapplications including operational amplifiers circuits and digital toanalog converters. There are a number of problems associated with suchdividers. For example, in thick film technology a voltage divider ismade by interconnecting two or more thick film resistors. All of theresistors must therefore be as nearly identical as possible, e.g. usethe same paste, same type of terminals and same fabrication techniquessimultaneously applied. Even then, one or more of the resistors willhave to be trimmed in order to accurately establish the correct voltageproportions at the taps. For a single tap, at least one but usually bothresistors are trimmed. For two taps, at least two resistors are trimmed,and so on. Trimming requires a first cut to bring the resistor withinabout one percent of the desired value, then heating the resistor tosoften a glass coating to relieve trim-induced stress, followed by asecond, smaller trim in the shadow of the first. Trimming affectstemperature stability to some extent, but affects long-term stability toan even greater extent. The approach of using a number of differentresistors requires close matching of the resistors to obtain similartemperature coefficients and long-term stability or driftcharacteristics among the resistors of a voltage divider.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improvedvoltage divider having one or more Kelvin connection taps.

It is a further object of this invention to provide such a voltagedivider which reduces tracking error over the full operating temperaturerange.

It is a further object of this invention to provide such a voltagedivider which reduces instability or drift due to aging and temperature.

It is a further object of this invention to provide such a voltagedivider which simplifies resistor fabrication and reduces fabricationtime and cost.

It is a further object of this invention to provide such a voltagedivider which can be trimmed precisely to the limits of the measuringequipment.

It is a further object of this invention to provide such a voltagedivider which enables a number of different ratios or taps to beindependently trimmed on a single resistor.

It is a further object of this invention to produce such a voltagedivider which does not require special or matched terminals or resistormaterial.

It is a further object of this invention to provide such a voltagedivider which does not rely on the absolute value of the resistance.

The invention results from the realization that in a Kelvin connectiontap laterally extending from a resistsor, a transverse cut or trim willadjust the effective location of the interconnection of the tap with theresistor without affecting the resistance or stability of the resistor.

The invention features a voltage divider having one or more Kelvinconnection taps. There are first and second spaced terminals and acurrent-carrying resistor between those terminals. A third terminal isspaced from the resistor. The resistor has at least one conductive,non-current-carrying peninsula extending from a junction on a side ofthe resistor and connecting with the third terminal to produce a Kelvinconnection tap. A cut is made only in the non-current-carrying peninsulaalong the path having a component parallel to the side of the resistorfrom which the peninsula extends and shading the junction of thepeninsula and the side from the third terminal to adjust theinterconnection of the third terminal with the resistor and vary thevoltage output without varying the resistance of the resistor. Anextension of this cut may be made parallel to the edge of the peninsulato further increase the adjustment. The peninsula may contain one ormore holes to increase the effectiveness of the adjustment.

The invention also features a method of making such a voltage dividerhaving one or more Kelvin connection taps, including first and secondspaced terminals and forming a current-carrying resistor between thefirst and second terminals. The method also includes constructing athird terminal spaced from the resistor and forming at least onenon-current-carrying peninsula of the resistor extending from a junctionon a side of the resistor and connecting it with the third terminal toproduce a Kelvin connection tap. The setting of the voltage divider istrimmed by cutting into only the non-current-carrying peninsula along apath having a component parallel to the side of the resistor from whichthe peninsula extends and shading the junction of the peninsula and theside from the third terminal to adjust the interconnection of the thirdterminal with the resistor and vary the voltage output without varyingthe resistance of the resistor. The resistor may include a plurality ofsuch non-current-carrying resistors and associated third terminals. Thepeninsulas may all extend from one side of the resistor, or they mayextend from both sides of the resistor. The peninsula may extend at anangle or generally perpendicular to the side of the resistor for itsentire length, or only for its initial portion, after which it extendsgenerally parallel or at some other transverse angle to the side of theresistor. The peninsula may take any desirable form, includingtriangular, wherein the base of the triangle is at the side of theresistor and the edges of the triangle converge toward the thirdterminal. The peninsula may contain one or more holes to increase theeffectiveness of the adjustment.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur from the followingdescription of a preferred embodiment and the accompanying drawings, inwhich:

FIG. 1 is a view of a prior art voltage divider;

FIG. 2 is a view of a voltage divider having a Kelvin connection tapaccording to this invention;

FIG. 3 is a view of a voltage divider similar to FIG. 2 with analternate arrangement of the Kelvin connection tap and showing atriangular hole to increase effectiveness of a trim cut;

FIG. 4 is a view similar to FIG. 2 with another alternative constructionof the Kelvin connection tap and showing a round hole to increaseeffectiveness of a trim cut;

FIG. 5 is a view similar to FIG. 2 with still another construction ofthe Kelvin connection tap;

FIG. 6 is a view similar to FIG. 2 with yet another construction of theKelvin connection tap;

FIG. 7 is a view similar to FIG. 2 showing multiple connection taps onone side of the divider; and

FIG. 8 is a view similar to FIG. 2 showing multiple connection taps onboth sides of the voltage divider according to this invention;

FIG. 9 is a view of a voltage divider according to this invention withmore than one current-carrying segment and non-current-carryingpeninsulas.

There is shown in FIG. 1 a typical prior art voltage divider 10 which isformed of two resistors, 12, 14, interstitially arranged betweenterminals 15, 16 and 18. Terminal 16 acts as a voltage divider tap.Although it is shown generally centrally located in FIG. 1, it may belocated anywhere between terminals 15 and 18. Because the primarycurrent-carrying resistor 20 of voltage divider 10 is actually formed oftwo spearate resistors 12 and 14, great care must be exercised in themaking and trimming of these resistors. The material, the size and theshape of resistors 12 and 14 must be carefully controlled. They must beformed and fabricated of at least similar materials and at the same timein order to minimize temperature coefficient and long-term instabilitydifferences. Because they are two different resistors, any change in theresistance of one of them due to temperature changes or aging must bematched by a change at the same rate and in the same direction in theother resistor in order that the voltage at tap 16 remain unchanged.Even after careful fabrication, resistors 12 and 14 are usually notexactly the desired resistance value, and so at least one, and usuallyboth of them, will have to be trimmed. This is done by making atransverse cut 22 to gradually increase the resistance of resistor 12toward the ultimate correct value. The first cut 22 will normally bringthe value to within one percent of the desired value. A second cut 24 isthen made, typically in the shadow of the first, which trims the valuemore gradually toward the desired value. When one resistor is trimmed itoften becomes necessary to trim the other resistor, and so similar cuts26 and 28 may be required for resistor 14. This trimming, or cutting,introduces its own instabilities due to the exposure of the resistormaterial and the heating which is required to soften the glass coatingto remove the stresses induced by the cutting. The control of theinstabilities, the matching of the resistors and the tailoring ortrimming of the resistance values of the resistors become more complexwhen additional resistors 30, 32 and 34, and associated terminals 36, 38and 40, are added to provide additional taps.

This invention permits the absolute value of the resistance to beignored in adjusting for a particular voltage ratio or output. Thetrims, or cuts, are not made into the main body of the resistor; thustrimming does not affect the resistance of the resistor and there is norequirement to repeatedly trim or heat the resistor. The resistorconsists of essentially only one resistor and so matching is automatic.The voltage divider according to this invention is thus much more stableand much more accurately trimmable than prior art resistors, whetherthick film, thin film, monolithic, bulk film or wirebound.

By way of comparison, normal carbon composition resistors are generallynot trimmable. They have a temperature coefficient of 1000 to 2000ppm/°C., a long-term stability of 20,000 ppm/year, and tracking of500-1,000 ppm/°C. Conventional thick film resistors are trimmable towithin about 0.1%. They have a temperature coefficient of 50-100ppm/°C., a long-term stability after trimming of 500-1000 ppm variationper year; and a tracking of 5-50 ppm/°C. Thin film resistors are alsotrimmable to 0.1%. They have a temperature coefficient of 20-60 ppm/°C.,a long-term stability after trimming of 200-400 ppm variation per year,and tracking of 2-6 ppm/°C. In contrast, resistor ratios made accordingto this invention are trimmable to more than 0.001% accuracy and appearto be limited only by the readout capability of the measuring equipment.The long-term ratio drift is less than 0.1 ppm/1Khr., and tracking isbetter than 1 ppm/°C.

There is shown in FIG. 2 a voltage divider 50 according to thisinvention having first and second terminals 52, 54 with a primarycurrent-carrying resistor 56 between them. Extending laterally out fromthe side 58 of resistor 56 is a peninsula 60 made of the same materialas resistor 56. Peninsula 60 interconnects with terminal 62 to form theKelvin connection tap 64. By Kelvin connection is meant one in which no,or essentially no, current flows. The current flow is generallylongitudinally through resistor 56 from terminal 52 to 54, or fromterminal 54 to 52, and although there is a slight bulging 66 of thecurrent flow path there is virtually no current flow through peninsula60 to terminal 62. The voltage output from tap 64 is simply and stablelyadjusted by making a cut 68 which has a component parallel to the side58 from which peninsula 60 extends. Cut 68, as shown, interrupts thepath between the lower portion 70, shown shaded, of terminal 62, and thecorresponding area 72 of resistor 56. This causes the path to beestablished only between terminal 62 and the portion 74 of resistor 56.Thus the connection of tap 64 has been moved upwardly on resistor 56 dueto cut 68. If cut 68 is made somewhat longer the tap will be moved upfarther. If cut 68 were made shorter the tap would be lower. The cut mayalso be made from the top of the peninsula down and may not only extendalmost to the opposite side of the peninsula, but may also then turntoward the body of the resistor and run parallel to the edge of thepeninsula, almost all the way to the body of the resistor. In this waythe voltage may be shifted over a wide range without affecting theresistance of resistor 56. This is so because peninsula 60, althoughmade of the same material as resistor 56, has no current flow in it andthus does not introduce resistance variation or stability problems inresponse to cut 68.

The particular shape and arrangement of the peninsula may be varied, asshown in FIG. 3, where divider 50a has a peninsula 60a with a firstportion 75 that extends transversely to the sides 58a and a secondportion 76 which is at right angles to portion 75 and generally parallelto resistor 56a. However, the cut or trim 68a must always be made sothat it shades the junction 78 where the peninsula 60a meets the side58a of resistor 56a. For this purpose, the cut 68a is made along a pathhaving a component parallel to the side 58a of the resistor 56 fromwhich peninsula 60a extends. For example, if the cut were made at 68aait would have no effect of shifting the voltage divider tap, for cut68aa alone leaves the entire junction area 78 unobscured or unblockedfrom terminal 62a. Likewise, the cut 68aaa would not perform thenecessary shading because terminal 62a could still "see" the entirelength of junction area 78.

Although thus far peninsulas 60 and 60a have been shown extendingperpendicularly to sides 58 and 58a, this is not a necessary limitationof the invention. For example, as shown in FIG. 4, peninsula 60b mayextend at an angle. The cut 68b is parallel to side 58b and junction78b, but it need not be; the cut may be made at an angle, such asindicated at 68bb, so long as it has a component which is parallel toside 58b and junction 78b. Peninsula 60b may be extended with a secondportion 76b, as shown, or in any other similar fashion. The cuts ortrims 68 must be made so as to shade the junction of the peninsula fromthe third terminal 62bb. A hole such as triangular hole 77a, FIG. 3, orcircular hold 77b, FIG. 4, may be used to increase the effectiveness ofthe trim cut. In FIGS. 2, 3 and 4, the peninsula is approximately equalin width to one third the length of the resistor. In those cases, thetrim cut in peninsula 60 produces up to approximately a 25% adjustmentof the tap voltage. However, with a wider peninsula 60c, FIG. 5, thevoltage tap may be varied up to 100%, or close to it; a narrowedpeninsula or longer resistor would allow a finer adjustment.

Although the peninsulas in FIGS. 2-5 have generally straight parallelsides, that is not a necessary limitation of the invention, as variousshapes may be used without departing from the scope of the invention.For example, as shown in FIG. 6, peninsula 60d with cut or trim 68d maybe triangular in shape with its edges 80, 82 converging toward terminal62d and its base aligned generally coincident with junction 78d.

The voltage divider according to this invention is not limited tosingle-tap constructions; it may have any number of desired peninsulas60e, 60ee, 60eee, FIG. 7, and associated third terminals 62, 62ee,62eee, and the taps may be arranged on either or both sides as shown inFIG. 8, where there are four Kelvin connection taps 64f, 64ff, 64fff and64ffff, extending from side 58f, and four additional Kelvin connectiontaps 64'f, 64'ff, 64'fff and 64'ffff extending from the other side 90.

A voltage divider according to this invention 50g, FIG. 9, may alsoinclude another current-carrying resistor 100 with bothcurrent-carrying, 56gg, 56ggg, 56gggg, and conductive butnon-current-carrying, 60gg, 60ggg, 60gggg, peninsulas, in addition tonone or one, 60g, or more such peninsulas on current-carrying resistor56g.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

What is claimed is:
 1. A voltage divider having at least one Kelvinconnection taps comprising:first and second spaced terminals; acurrent-carrying flat resistor between said first and second terminals;a third terminal spaced from said primary resistor; said resistor havingat least one conductive, non-current-carrying peninsula of similarresistive material extending from a junction on a side of said resistorand connecting with said third terminal to produce a Kelvin connectiontap; and a cut only in said non-current-carrying peninsula along a pathhaving a component parallel to said side of said resistor from whichsaid peninsula extends and shading the junction of said peninsula andsaid side from said third terminal to adjust the interconnection of saidthird terminal with said resistor and vary the voltage output withoutvarying the resistance of the resistor.
 2. The voltage divider of claim1 in which said resistor includes a plurality of non-current-carryingpeninsulas and associated third terminals.
 3. The voltage divider ofclaim 2 in which said peninsulas all extend from one side of saidresistor.
 4. The voltage divider of claim 2 in which said peninsulasextend from both sides of said resistor.
 5. The voltage divider of claim1 in which said peninsula extends generally perpendicular to said sideof said resistor for its entire length.
 6. The voltage divider of claim1 in which said peninsula extends generally perpendicular to said sideof said resistor for the initial portion of its length and the remainingportion extends generally parallel to said side.
 7. The voltage dividerof claim 1 in which said peninsula is triangular in shape with its baseat said resistor and its edges converging toward said third terminal. 8.The voltage divider of claim 1 in which said peninsula has a hole in it.9. The voltage divider of claim 1 in which said voltage divider has aplurality of current-carrying resistors, at least one of which includesa conductive, non-current-carrying peninsula.